Yes, I know that can be very annoying. It's a mistake on the parent's parts. Here, we have a small walk/skate/cycling park. Walkers, skaters, and cyclists have their own lanes, which are nicely pictured on the pavement. I HATE when people go into a lane where they aren't supposed to be. And, these are grown-ups. They see a fast cyclist coming, but they continue to weave in and out of their and my lane.
I'd say that if it happened again, just ask the parents to tell their children to watch out for you, since they can't figure it out on their own.